This invention relates to the medical field of orthopaedics and joint replacement, in particular. Modular artificial joints have several components that must be assembled and placed in the patient to reconstruct a joint. While modular joints provide the ability to custom fit an artificial joint to a patient""s anatomy, the connection between the components must be without relative movement after implantation. This invention is directed to a modular artificial joint construction which provides a locking mechanism to secure the components immovably together.
Artificial joints or prosthesis have now been constructed for almost every natural joint in the living body. As the medical field gains more understanding of the problems involved in mating inanimate constructions with animate tissue and designing mechanical devices that can duplicate natural movement, the number of implantations will continue to increase. In addition to the major joints, such as the hip, knee, shoulder, elbow, wrist and ankle, better engineering of the prosthesis, accompanied with miniaturization, will permit smaller and smaller natural joints, e.g. vertebrae, phalanges and tarsals, to be reconstructed. Until now, the larger joints have received the most attention mainly because of the larger size of the bones. The prosthesis of this invention may be utilized in all joints.
In replacing a hip joint, the head of the femur is removed along with the ball. The trochanter portion of the femur is shaped and prepared for receiving the prosthesis so that the artificial joint will closely approximate the natural hip.
Earlier artificial hip joints were made of one-piece construction requiring a large inventory of prosthesis to accommodate the various sized patients. The modular artificial joint has two or three or more elements which replace the natural hip. By manufacturing these components with interchangeable connections but different external sizes, inventories may be smaller because of the ability to mix and match components. Also, the modular prosthesis provides more flexibility in customizing the various components of a joint to the various parts of a patient""s natural joint.
In a three piece artificial hip joint, the various sized components of the joint that may be selected are the intramedullary rod, the trochanter and the neck. The intramedullary rod is inserted into the end of the femur. The rod acts as a stabilizer in maintaining the artificial joint in the axis of the femur. The upper portion of the rod which extends out of the femur is fitted into a trochanter element which is shaped like the removed broad head of the femur which it replaces. This element, along with the rod, is used to adjust the length of the prosthesis to approximate the natural length of the femur.
The natural trochanter is the broadened area offset from the end of the femur. The natural trochanter may be at any radial angle about the axis of the femur. This natural angular relationship must be reproduced by the intramedullary rod and the artificial trochanter. The artificial trochanter is seated on the end of the patient""s femur and is the main load bearing element of the prosthesis. It is important that this load, which is mostly compression, is transmitted along the axis of the femur.
A neck element is inserted into the trochanter element and carries an extension onto which the ball joint will be fixed. The horizontal angle between the trochanter and the neck extension is variable to reproduce the anteversion angle of the patient""s natural joint. The neck carries cantilevered forces in torque and compression between the acetabulum and the trochanter. It is also important that these forces do not result in relative movement between the trochanter and the neck.
All these elements have a central bore and are permanently secured together by a bolt which is inserted into the neck element, extends through the trochanter element, and is threaded into the upper end of the rod. In some cases, the intramedullary rod may be attached to the bone with bone cement while, in other cases the cement is omitted.
When the cement is omitted, the placement and fixation of the intramedullary rod becomes more critical to pain free usage of the prosthesis. Further, it is most important that the intramedullary rod not be disturbed after insertion since this would corrupt the union between the rod and the interior of the femur.
In order to maintain the original union between the femur and the intramedullary rod, modular prosthesis have been developed to allow rotational adjustment of the several parts or elements about the emplaced rod during the placement of the prosthesis to more closely reproduce the natural structure of the hip. It has been found that, in some cases, as the intramedullary rod has been inserted into the bone canal, there is rotational movement of the rod. In order to preserve the union between the rod and the bone, there must be a mechanism to accommodate the changed angular orientation of the proximal end of the intramedullary rod so that the prosthesis closely approximates the natural trochanter and ball.
While the above description refers to a modular hip prosthesis, substantially the same considerations must be given to other modular prosthesis, such as a knee prosthesis in which an intramedullary rod is placed in the lower end of the femur and in the upper end of the tibia or the elbow in which an intramedullary rod is placed in the lower end of the humerus and the upper end of the radius or ulna. Because of individual physical anomalies, the functional prosthesis must be capable of angular adjustment to conform to the natural physique.
With the advantage of flexibility gained by modular prosthesis, there comes the requirement that there be no movement between the several parts or elements after implantation. These movements may cause misalignment of the joint resulting in increased pain, trauma to the joint and, even, dislocation of the joint.
The prior art is replete with artificial prosthesis and hip joints, in particular.
Illustrative of the state of the art is U.S. Pat. No. 5,876,459 to Powell which discloses a modular hip joint having a stem, one end of which is inserted in the intramedullary canal. The other end of the stem is tapered to fit within a second, neck, element. The neck ultimately supports the ball joint. A sleeve element is placed over the junction of the first two elements. All three elements are rotationally movable relative to each other. A bolt is driven through the bore of the neck and stem deforming a portion of the interconnected elements for a friction fit between the neck and the stem. These prior art patents disclose that the sleeve may have a polygonal shaped bore with the articulating elements having corresponding shaped portions. The interconnected elements of these hip joints do not form a static lock between each other but require a deformation of one or more elements before a friction fit is established. The deformation and friction fit is between the stem and the neck rather than the sleeve and the stem.
U.S. Pat. No. 5,653,765, to McTighe et al discloses a modular hip joint with a stem, an intermediate shoulder portion, and a proximal shoulder piece which attaches to the ball. The stem and the intermediate shoulder portion have interengaging teeth on the corresponding ends of each by which they are connected. This end-to-end connection allows for rotational movement of the elements relative to each other. The proximal shoulder piece and the intermediate shoulder piece also have an end-to-end toothed connection for rotational adjustment. This construction has two movable end-to-end connections which provide good flexibility for rotation of the elements but have small surface areas of fixation to each other limited to the surfaces of the interengaged teeth.
The Leto patent, U.S. Pat. No. 4,419,026 issued Dec. 6, 1983, discloses a resilient split sleeve camming lock for use with telescoping tubular elements. The system relies on the resilience of the split ring and does not require a permanent deformation of the split sleeve by longitudinal displacement.
In the instant invention a modular prosthesis is taught which has an intramedullary rod element which is to be inserted in a bone. The rod has a shaped proximal portion which is telescoped into one end of a bore in the weight-bearing element which is the artificial trochanter in the hip prosthesis. The mating surfaces of the shaped rod and the weight-bearing element bore are shaped to permit 360xc2x0 rotation of the rod within the bore. The wall shapes of the proximal portion of the rod and the bore are complimentary but of different sizes to accommodate a split sleeve disposed between the rod and the bore. This mechanism allows the trochanter to be rotated on the distal end of the intramedullary rod without disturbing the placement of the rod in the intramedullary canal.
Upon relative longitudinal movement between the bore and the rod a rotationally immovable connection is formed between the intramedullary rod and the weight-bearing element or trochanter. The artificial ball element is telescoped into the other end of the trochanter bore permitting additional rotational adjustment. All the elements are locked together by a bolt through the neck and rod.
In a particularly preferred embodiment of the instant invention a modular prosthesis is described for use as a joint prosthesis having an intramedullary rod, a weight-bearing component and a artificial ball. The intramedullary rod has a distal end adapted for insertion into the intramedullary canal of the femur and a proximal end. The proximal end includes a screw threaded throughbore along the longitudinal axis of the intramedullary rod.
The weight-bearing component has a narrow distal end and a larger proximal end forming an external shape approximating the natural bone. The weight-bearing component has a through bore from the distal end to the proximal end, with the proximal end of the through bore having a smooth circumference. The distal end of the through bore has a circumference with opposite planar sides joined by curved surfaces. The circumference of the trochanter bore and the circumference of the proximal end of the intramedullary rod telescope together with the opposite planar surfaces in intimate contact with each other forming a rotationally secure connection with the artificial trochanter approximating the position of the natural trochanter.
The ball element has a planar distal end with a through bore. There is a cylindrical extension about the through bore adapted to be inserted into the proximal end of the through bore of the artificial trochanter. The extension and the wall of the trochanter bore may have complimentary shapes to interlock without rotational movement. Alternatively, there may be a key lock formed as a pin fitting into an aperture on the opposing contacting surfaces of the ball element and the trochanter. The proximal end of the through bore in the neck has an enlarged countersunk bore and the distal end of the through bore telescopes over the proximal end of the intramedullary rod. A screw threaded bolt is disposed in the countersunk bore and threadably engaged with the screw threads in said proximal end of said intramedullary rod forming a locked integral prosthesis.
Accordingly, it is an objective of the instant invention to provide a joint with an intramedullary rod element which is connected with the weight-bearing element in such a manner as to provide infinite rotational adjustment therebetween. Rotational movement, in this context, refers to the turning of either element in a plane normal to the common longitudinal axis of the elements.
It is another objective of the instant invention to provide a locking mechanism between the intramedullary rod and the weight-bearing element to permanently fix the components together after rotational adjustment.
It is a further objective of the instant invention to provide a connection between the trochanter element and the intramedullary rod in such a manner as to limit the combined length of the elements.
It is a further objective of the instant invention to provide the trochanter and the neck with a locking mechanism to rigidly secure the components together to prevent relative rotation.
It is a still further objective of the invention provide a locking mechanism between the neck element and the trochanter element that permits rotational adjustment of the anteversion angle.